This invention relates to a composite article formed of a nonfoam region and a ceramic foam region.
In many applications, the property requirements of an article vary greatly according to location within the article. In one example, some locations within the article must have excellent mechanical properties at high temperatures and other locations within the article function largely to define the form factor (i.e., the shape) of the article and have much lower requirements for their mechanical properties. In most cases, the different property requirements are met with a single material of construction that may not be optimal for any one location but instead achieves a good balance of properties for all of the locations.
Composite materials have been developed for use at room temperature and mildly elevated temperatures. Such composite materials include the familiar fiber-reinforced organic matrix composites such as graphite fiber-epoxy composites. Structures made of such materials may have their properties tailored according to the location within the article, by changing the direction of the fibers, the volume fraction of the fibers, the type of fibers, and the like.
There have been attempts to apply these principles of composite construction to high-temperature applications. For example, components of gas turbine engines have widely varying property requirements between relatively closely spaced locations. A low-pressure gas turbine blade must be strong in the dovetail and root sections, but much of the airfoil serves largely to define a shape that is mechanically stressed to only a modest level. The mechanical properties must be retained to elevated temperatures, inasmuch as the efficiency of the engine increases with increasing operating temperature. There is a large incentive to raise the combustion gas temperature of the engine.
However, there is also a large incentive to decrease the weight of the turbine blade as much as possible, because a reduction in turbine blade weight leads to reductions in disk weight, shaft weight, bearing weight, and support weight that in turn increase the weight efficiency of the engine.
Research studies have been underway for many years to apply composite-construction principles to high-temperature components such as turbine blades. These efforts have focused on superalloys that are reinforced by particles, fibers, or whiskers of ceramic materials. Although there have been some advancements, these efforts have not been successful in the sense that there are no such composite articles in regular service today. Gas turbine blades are typically made of nickel-base superalloys that may be made hollow to reduce weight and to allow cooling air to be conveyed through the interior of the blades. The use of a composite construction would offer the promise of reducing weight while maintaining or improving performance, but no operable approach has been proposed as yet.
There is, accordingly, a need for an improved approach to articles that must operate at elevated temperatures, must have property requirements that vary substantially at different locations of the article, and must be as light in weight as possible. The present invention fulfills this need, and further provides related advantages.
The present invention provides a composite construction that is applicable to articles which operate at high temperatures. The structure utilizes a combination of metallic regions and ceramic foam regions to tailor the properties as required for excellent mechanical properties and for low weight. The approach of the invention allows the designer of the article to determine the required combination of properties in each location, and then the article is manufactured with different materials optimized for each location.
An article of manufacture comprises a metallic nonfoam region, and a ceramic foam region joined to the metallic non foam region. The ceramic foam region comprises an open-cell solid ceramic foam made of ceramic cell walls having intracellular volume therebetween. The ceramic cell walls are preferably alumina. The intracellular volume may be empty porosity or an operable intracellular metal such as an intracellular nickel-base superalloy. The ceramic foam region may even be varied within itself, to have a first ceramic foam subregion having an intracellular volume that is empty porosity, and a second ceramic foam subregion having an intracellular volume comprising the intracellular metal. The metallic nonfoam region may be any operable metal, such as a primary nickel-base superalloy. The nonfoam region and the ceramic foam region are joined by any operable approach, such as a diffusional joint or a casting joint.
In one approach, a method of preparing an article comprises the steps of preparing a ceramic foam region by the steps of providing a piece of a sacrificial ceramic having the shape of the ceramic foam region, and contacting the piece of the sacrificial ceramic with a reactive metal which reacts with the sacrificial ceramic to form an oxidized ceramic of the reactive metal and a reduced form of the ceramic. The resulting structure comprises a ceramic foam of the oxidized ceramic compound of the reactive metal with ceramic cell walls and an intracellular volume between the ceramic cell walls, the intracellular volume comprising a reaction-product metal. The reaction-product metal may be removed to create empty porosity, or replaced with a replacement metal. The ceramic foam region is joined to a metallic nonfoam region, as by interdiffusing the two regions or casting the metallic nonfoam region around the ceramic foam region.
The present approach provides a great deal of flexibility in precisely tailoring an article that is to be used at high temperatures. The ceramic foam material is lighter in weight than a comparable superalloy, and the weight may be reduced even further by removing the reaction-product metal from the intracellular volume where mechanical property requirements are minimal and the material functions largely to define a form. Where the mechanical property requirements are higher, the reaction-product metal may be replaced with the intracellular nickel-base superalloy to produce a ceramic foam whose intracellular volume is filled with the superalloy.
The joining of the ceramic foam regions and the nonfoam regions is accomplished by any operable approach. In one technique, the regions are each fabricated separately and then joined by solid-state joining, liquid-phase joining that may be possible in some cases, or brazing with a brazing metal. In another technique, the ceramic foam region is fabricated, and the metallic nonfoam region is cast around it.